Colloidal photonic crystals

ABSTRACT

A method of growing robust large area colloidal photonic crystals and devices produced thereby. A suspension of monosized colloidal spheres ( 1 ) is subjected to a composite shear ( 6 ) by applying a sequential set of shearing forces. The crystalline layers within the colloid experience shearing forces with components in both x and y directions, forcing the colloid into a singe face-centered-cubic structure in preference to a twinned face-centered-cubic structure. The method may also comprise the use of a dispersion medium which is capable of undergoing a controllable phase change from a liquid phase to a solid phase. The crystal may be fixed into a single face-centered cubic structure.

This application is the US national phase of international applicationPCT/GB00/03405 filed 5 Sep. 2000, which designated the US.

The present invention relate to colloidal photonic crystals, to a methodof growing robust large area colloidal crystals and devices producedthereby.

For the purposes of this patent, a photonic crystal shall be defined asan object whose optical properties are spatially periodic.

It has been known for some time that colloidal suspensions can be madeto crystallise under certain conditions to produce colloidal crystalswhich exhibit interesting optical properties.

Such photonic colloidal crystals are capable of modifying thepropagation of light due to the fact that the crystal structure isperiodic on the scale of the wavelength of light. Accordingly, colloidalphotonic crystals find applications in a variety of optical devicesincluding optical filters and limiters. The reflective properties ofcolloidal photonic crystals can also be controlled offering furtheropportunities for exploitation in optical devices.

Bulk samples of such crystals are however usually polycrystalline andcomprise many hundreds of crystals of the order of 100 microns in sizewhich are randomly oriented. The crystals may also possess a variety ofcrystalline structures including face-centred-cubic (fcc),hexagonally-close-packed (hcp), and random-close-packed (rcp). These andother imperfections within the crystal impair the opticalcharacteristics of the crystal and make the crystal unsuitable formaterials applications.

An improved method for growing colloidal photonic crystals has beenproposed by P. N. Pusey and B. Ackerson, in patent GB 2 260 714. Theimproved method reduces the imperfections in the crystal by melting andaligning the crystal into a preferred structure. Specifically, thismethod relates to a suspension of monosized polymer colloidal spheresand consists of aligning the colloid into a face-centred-cubic crystalstructure by applying a rectilinear shearing force usually by inducingflow in the liquid.

The photonic crystals formed by this method are essentially perfectface-centred-cubic structures. The method enables single crystals to begrown over areas larger than 1 cm².

At the end of the growth process, after sufficient shearing force hasbeen applied to establish a substantially single crystal structure, thestructure may be sealed to retain the carrier liquid. Alternatively,some form of gelling agent may be added to the carrier liquid to improvethe stability of the structure or the carrier liquid may be allowed toevaporate to a leave a self-supporting structure of colloidal particles.

Shearing produces two types of single face-centred-cubic structure, onetype being produced on the forward shear, the second type being producedon the reverse shear. When the shearing is stopped the colloid relaxesinto a twinned face-centred-cubic structure. In the case of the twinnedarrangement both forms of face-centred-cubic structure coexist withinthe crystal, one on top of the other.

Whilst the twinned face-centred-cubic structure exhibits useful opticalcharacteristics, the single face-centred-cubic arrangement providesimproved optical properties over the former. For example, the singleface-centred-cubic structure demonstrates improved photonic band-gapproperties and can be optimised to be reflective for a large range ofangles of incident radiation and polarisation angles (for incidentradiation within a limited wavelength range); see for exampleYablonovitch et al, “Three-dimensional photonic band structure”,[(Yablonovitch, E., Gmitter, T. J., Leung, K. M., Meade, R. D., Rappe,A. M., Brommer, K. D., Joannopoulos, J. D., “Three-dimensional photonicband structure”, Opt. & Qu. Elect., 24, S273, 1992] and referencestherein. The single face-centred-cubic structure therefore offers agreater potential for high quality optical devices but cannot be made bythe aforementioned linear shearing method because of the tendency of thecrystal to relax into the twinned face-centred cubic structure.

Further limitations of current colloidal photonic crystals relate to thephysical properties of the crystals. Current colloidal photonic crystalsare relatively fragile and lack permanence, largely due to the fact thatthe crystalline layers are merely held in place between rigid parallelplates. With reference to configurations in which the carrier fluid isretained within the colloidal crystal, the sealing can becomecompromised allowing unwanted evaporation of carrier liquid leading todegradation of the crystalline structure. Further, the physicaldimensions of the crystals remain relatively small precluding widespreadadoption of colloidal photonic crystals in optical applications.

It is an object of the present invention to provide an improved methodfor producing robust large area colloidal photonic crystals.

According to the present invention, a method of growing an essentiallyperfect colloidal photonic crystal exhibiting a singleface-centred-cubic structure comprises the steps of:

preparing a suspension of monosized colloidal spheres having a volumeconcentration that produces spontaneous local crystallisation in asuitable dispersion medium,

inserting the colloidal suspension into a gap between two substantiallyparallel surfaces,

subjecting the surfaces to relative oscillating motion parallel to theirsurfaces and,

subjecting the surfaces to a series of small linear displacementsrelative to each other, the displacements being parallel to theirsurfaces and in two dimensions, comprising the sequence of applying alinear displacement to one of the surfaces with respect to the othersurface, rotating the direction in which the linear displacement isapplied to the surface by substantially 120 degrees in a single constantdirection and applying a further linear displacement to the surface, thesequence being repeated until the colloidal photonic crystal has beenpurified into a single face-centred-cubic structure.

Preferably the dispersion medium is one that can be changed from aliquid phase to a solid phase in order to fix the colloidal crystallinestructure.

The direction of rotation may be either clockwise or anticdockwise.

In an another arrangement, the method of growing an essentially perfectcolloidal photonic crystal exhibiting a single face-centred-cubicstructure comprises the steps of:

preparing a suspension of monosized colloidal spheres having a volumeconcentration that produces spontaneous local crystallisation, in adispersion medium that can be changed from a liquid phase to a solidphase in order to fix the colloidal crystalline structure

inserting the colloidal suspension into a gap between two substantiallyparallel surfaces, and

subjecting the surfaces to relative oscillating motion parallel to theirsurfaces.

In particular, the magnitude of the small linear displacements appliedto the surfaces is substantially equal to the product of the diameter ofthe colloidal spheres and the number of crystalline layers in thecrystal.

In one preferred arrangement the surfaces are displaced with respect toeach other in an equilateral triangle.

The minimum volume fraction of monosized colloidal spheres isadvantageously 0.49 and preferably the radius of the monosized colloidalspheres is in the range 0.01 μm to 100 μm.

Preferably the radius of the monosized colloidal spheres is in the range0.05 μm to 10 μm.

The colloidal spheres may comprise at least one of a polymer, anonlinear material, a magnetic material, a metal, a semiconductor, glassdoped with an active dye, polymer doped with an active dye, and silica.In particular the colloidal spheres may be polymethylmethacrylate.

The material used for the dispersion medium is preferably at least oneof an adhesive, a polymer, a resin, a non-linear optical material, anactive optical material, octanol.

In a preferred embodiment the active optical material used for thedispersion medium is a liquid crystal material.

In one arrangement the dispersion medium is subsequently removed fromthe colloidal photonic crystal to leave a structure comprising colloidalspheres surrounded by an interconnecting matrix of voids. A substitutematerial may be subsequently introduced into the interconnecting matrixof voids surrounding the colloidal spheres. The substitute material maybe at least one of a metal, a semiconductor, a nonlinear opticalmaterial, an active optical material.

In a preferred embodiment the colloidal spheres may be subsequentlyremoved from the substitute material to produce an inverse singleface-centred-cubic structure.

In a further preferred embodiment the active optical material used forthe substitute material is a liquid crystal material.

Where the dispersion medium or the substitute material is a liquidcrystal, means for applying an electric field to the liquid crystalmaterial may be added to the colloidal photonic crystal.

In a preferred embodiment, the dispersion medium is an epoxy resin andthe method of growing an essentially perfect colloidal photonic crystalexhibiting a single face-centred-cubic structure further comprises thesubsequent step of curing the resin to form a solid interconnectingmatrix between the colloidal spheres.

The curing process preferably includes at least one of exposure toelectromagnetic radiation, exposure to ultraviolet radiation, chemicalreaction, elevation of temperature.

At least one of the substantially parallel surfaces may comprise asubstantially flexible membrane.

In a preferred embodiment bulk colloidal photonic crystal film may beproduced by applying the linear displacements to the parallel surfacesby rolling means.

In a further preferred embodiment the method further includes theintermediate step of applying a detachable membrane to the internal faceof at least one of the parallel surfaces prior to Introducing thecolloidal suspension.

The internal surface of at least one of the parallel surfaces may betextured to promote the growth of multiple crystal domains.

The refractive index of the dispersion medium may be substantiallydifferent from the refractive index of the colloidal spheres. Further,the refractive index ratio between the colloidal spheres and thedispersion medium may greater than two.

In one preferred arrangement, the method comprises the subsequent stepof removing the colloidal spheres from the solidified dispersion mediumto produce an inverse single face-centred-cubic structure.

In a further preferred arrangement the method further comprises thesubsequent step of introducing a substitute material into the voidscreated in the solidified dispersion medium by the removal of thecolloidal spheres, thereby producing a substituted singleface-centred-cubic structure. The substitute material may be anon-linear optical material, an active optical material or a laser dye.

In a preferred arrangement the two surfaces used to retain the colloidalsuspension may be concentrically cylindrical.

In a second aspect of the invention there is provided a colloidalcrystal grown according to the above method.

In one preferred arrangement the colloidal crystal forms an opticalnotch filter wherein the colloidal sphere radius and refractive index ofthe dispersion medium are selected to co-operate to reflect at least onespecific wavelength and to transmit other wavelengths.

In a further preferred arrangement the colloidal crystal is incorporatedin an optical device which further comprises a liquid crystal materialand means for applying an electric field to the liquid crystal material,wherein a variable voltage may be applied to the liquid crystal materialto change the refractive index contrast between the liquid crystalmaterial and the colloidal spheres.

The invention will now be described, by example only, with reference tothe accompanying drawings in which;

FIG. 1 illustrates the conventional method for growing colloidalphotonic crystals by applying a linear shearing force to a suspension ofmonosized polymer colloidal spheres,

FIG. 2 shows a diagram of the arrangement used to characterise thestructure of the colloidal photonic crystal,

FIG. 3 shows schematic representations of the three differentdiffraction patterns produced during the growth process. Specifically,FIGS. 3a and 3 b illustrate the diffraction patterns produced by the twoforms of single face-centred-cubic crystals respectively, whilst FIG. 3cshows the diffraction pattern formed at the start of the growth processby the twinned face-centred-cubic configuration,

FIG. 4 shows graphs of intensity of diffracted radiation versus angle ofincidence (θ) for the various crystalline structures produced during thegrowth process,

FIG. 5a illustrates the improved method for producing colloidal photoniccrystals by applying a composite two dimensional shear to the colloidalcrystal,

FIG. 5b illustrates the improved method for producing colloidal photoniccrystals by applying a composite two dimensional shear to the colloidalcrystal in a cylindrical geometry,

FIG. 6a Illustrates the Iterative steps within the composite shearingprocess and the directions in which successive shears are applied to thecrystal,

FIG. 6b illustrates the specific case where the composite shear processis an equilateral triangle,

FIG. 7 illustrates schematically a technique for large scale productionof single face-centred-cubic colloidal photonic crystals using thecomposite two-dimensional shear method.

Referring to FIG. 1, the conventional method for growing colloidalcrystals utilises a suspension of monosized polymer colloidal spheres(1) placed between two glass plates (2,3). Typically the colloidconsists of 800 nm polymethylmethacrylate spheres suspended in octanolat a volume concentration that produces spontaneous crystallisation (inpractice greater than 49% by volume). The two glass plates (2,3) arecleaned and 10 micron spacer beads (4) are spread onto one of theplates. The colloid suspension (1) is placed on one plate and the otherplate placed on top and pressure is applied so that the space betweenthe plates is completely filled. At this point the sample ispolycrystalline comprising randomly oriented small crystals.

The sample is now sheared by applying an oscillating linear lateraldisplacement (5) to the top plate (2) with respect to the lower plate(3). The linear lateral oscillations are confined to a single direction,denoted in the figure as the x-direction, and have an amplitudeapproximately equal to ten times the gap between plates (2) and (3).This melts the crystal and aligns the colloids into hexagonally closepacked layers.

The sample is then sheared by linear oscillations of a smaller amplitudeto force the colloids into a face-centred cubic structure. The amplitudeof the oscillations is typically equal to the product of the diameter ofthe colloidal spheres and the number of crystalline layers in thecrystal. Importantly one face-centred cubic structure is produced on theforward shear stroke and another face-centred cubic structure isfavoured on the backward stroke. If the shearing is stopped the colloidrelaxes into a twinned face-centred cubic structure, both forms offace-centred cubic structure coexisting one on top of the other.

Referring to FIG. 2. the optical properties of the colloidal crystaldepend on the crystal structure and this phenomenon can be used tocharacterise the structure of the colloidal crystal produced. The samplecrystal (10) is illuminated with a laser beam (11) and the intensity ofthe diffracted spots (12) is measured as a function of the angle of theincident beam θ (13). During the shearing operation a differentdiffraction pattern is produced by each of the two types of singleface-centred-cubic structure.

On the forward shear stroke the first single face centred-cubicstructure produces a diffraction pattern comprising three Bragg spotsspaced 120 degrees apart, as shown in FIG. 3a. On the reverse shearstroke the second single face-centred-cubic structure produces adifferent diffraction pattern of three different Bragg spots spaced at120 degrees apart, as shown in FIG. 3b. Upon removing the linearshearing. force, the colloid relaxes into a twinned face-centred-cubicstructure and all six Bragg spots are visible, as shown in FIG. 3c.

Referring to FIG. 4, the graphs of intensity of diffracted radiationversus angle of incidence (θ) illustrate the spatial distribution andintensity of the various crystalline structures produced during theshearing process. The intensity and spatial distribution of thediffracted beams are important parameters in determining the crystallinestructure and have an impact on the performance any potential opticaldevice incorporating the colloidal photonic crystal. FIG. 4a illustratesthe response from the random close pack structure prior to applying anyshearing force to the sample. FIG. 4b shows the response from thehexagonal close packed crystal. FIGS. 4c and 4 d show graphs ofintensity versus incident angle for the two single face-centred-cubicstructures respectively.

The tendency of the colloidal crystal to relax into a twinnedface-centred-cubic structure may be eliminated by using an enhancedshearing process as illustrated in FIGS. 5a and 5 b.

Referring to FIG. 5a, the enhanced method of producing singleface-centred-cubic crystals utilises a suspension of monosized polymercolloidal spheres (1) placed between two glass plates (2,3). The colloidconsists of 800 nm polymethylmethacrylate spheres suspended in octanolat a volume concentration that produces spontaneous crystallisation(greater than 49% by volume). The two glass plates (2,3) are cleaned and10 micron spacer beads (4) are spread onto one of the plates. Thecolloid suspension (1) is placed on one plate and the other plate placedon top and pressure is applied so that the space between the plates iscompletely filled. A polycrystalline sample is obtained comprisingrandomly oriented small crystals.

The sample is now sheared by applying an oscillating linear lateraldisplacement (5) to the top plate (2) with respect to the lower plate(3). The linear lateral oscillations are confined to a single direction,denoted in FIG. 5 as the x-direction. The linear lateral oscillationsmay typically be of an amplitude equal to ten times the gap betweenplates (2) and (3). This melts the crystal and aligns the colloids intohexagonally close packed layers.

The sample is then subjected to a composite shear (6) by applying asequential set of shearing forces to the sample. The sequential set ofshearing forces causes the top plate (2) to undergo a series of linear,two dimensional displacements with respect to the lower plate (3). Thecrystalline layers within the sample experience shearing forces withcomponents in both x and y directions, forcing the colloid into a singleface-centred-cubic structure and ensuring the single face-centred-cubicstructure is maintained in preference to the twinned face-centred-cubicstructure.

The composite shearing process is illustrated in FIG. 6a. Specifically,the composite process consists of rotating the direction in which theshear force is applied to the crystal by substantially +120 degrees(angle ø) between successive shears in the plane of the substrate. Thisproduces the first form of the single face-centred-cubic crystallinestructure.

Alternatively, the direction in which the shear force is applied to thecrystal is rotated by substantially −120 degrees (angle ø) betweensuccessive shears in the plane of the substrate. The alternative form ofthe single face-centred-cubic crystalline structure is produced withinthe crystal.

The successive linear displacements do not have to be equal magnitude,although a threshold exists above which the crystal structure melts andthe colloids realign into hexagonally close packed layers. The magnitudeof each linear displacement is typically equal to the product of thediameter of the colloidal spheres and the number of crystalline layersin the crystal. The process is repeated until all the crystalline layershave been sheared into a single face-centred-cubic structure. Thecrystalline structure can be confirmed by illuminating the sample with alaser beam and measuring the intensity of the diffracted Bragg spots asa function of the angle of the incident beam (FIG. 2 refers).

Typically, the path described by the top plate (2) during the compositeshear process is an equilateral triangle as shown in FIG. 6b.

Cylindrical geometries may also be used to implement the compositetwo-dimensional shear as shown in FIG. 5b. Referring to FIG. 5b, thesuspension of monosized polymer colloidal spheres (1) is placed betweencylindrical plate (20) and rod (21), which are concentric. The sample isnow sheared by applying an oscillating rotary motion (22) to the rod(21) about axis A (23). This melts the crystal and aligns the colloidsinto hexagonally close packed layers.

The sample is then subjected to the composite, two-dimensional shearingforce by translating the rod in an oscillatory manner (24) along thex-axis whilst continuing to apply the oscillating rotary motion (22)about axis A (23), such that the two motions combine to rotate thedirection in which successive shears are applied to the crystal bysubstantially 120 degrees. The colloid is forced into either the firstor second form of the single face-centred-cubic crystal depending uponthe direction of rotation of the shear force between successive shears.

The single face-centred-cubic crystalline structure produced by theenhanced composite shearing technique, using either linear orcylindrical geometries, may be further processed to modify theproperties of the structure.

At the end of the growth process the dispersion medium may be removedfrom the colloidal crystal structure to leave a self supportingstructure of colloidal spheres surrounded by an interconnecting matrixof voids. For the purpose of this specification the term void shall notbe limited to cavities or spaces containing a vacuum but shall includecavities or spaces containing any combination of any gases, includingair. The interconnecting matrix of voids may be subsequently filled withother materials to modify the optical properties of the photoniccolloidal crystal.

The materials used to fill the interconnecting matrix of voids typicallyinclude metals or semiconductors, for example silicon, germanium orgallium arsenide.

Alternatively the interconnecting matrix of voids produced by theremoval of the dispersion medium from the photonic colloidal crystal maybe filled with an active material such a liquid crystal material. Anematic liquid crystal material may typically be used. Using a liquidcrystal material in place of the dispersion medium in the photoniccolloidal crystal enables the refractive index contrast between thecolloidal 10 spheres and the dispersion medium to be controlled, therebyproviding control over the diffractive and bandgap properties of thephotonic crystal.

The refractive index contrast in the photonic crystal may be modified byvarying the temperature of the liquid crystal material or by applying avariable electric field across the liquid crystal material in thephotonic crystal.

Under low intensity illumination the nematic liquid crystal and thecolloidal spheres are index matched and the crystal is substantiallytransparent. If the glass plates containing the colloidal photoniccrystal are anti-reflection coated, transmission through the device mayexceed 90%. Increasing the temperature of the liquid crystal materialcauses the liquid crystal to re-orientate and the liquid crystalmolecules change from a nematic phase to an isotropic phase. Thisresults in an increase in the refractive index contrast between theliquid crystal material and the colloidal spheres. Thus the colloidalphotonic crystal now operates as a diffraction grating and transmissionthrough the device decreases. If the wavelength of radiation incident onthe colloidal photonic crystal corresponds to the position of the bandgap in the photonic crystal then the transmission through the device isgreatly reduced.

The temperature of the liquid crystal material may be varied by heatingdirectly the bulk colloidal photonic crystal. Altematively local heatingof the liquid crystal material may be brought about by illumination witha high intensity pulse. Absorption of the high intensity pulse by thecolloidal photonic crystal (both by the colloidal spheres and the andthe liquid crystal material) giving rise to local heating of the liquidcrystal material.

Laser dyes may be also be incorporated to enhance this effect and tomake the colloidal photonic crystal sensitive to specific wavelengths.In this case a laser dye is dissolved into the liquid crystal. Anincident beam, of a wavelength which lies within the absorption band ofthe dye, absorbs energy from the beam giving rise to local heating asbefore, but now the transfer of energy from the incident beam to thecrystal is significantly larger due to the increased absorptioncross-section of the laser dye. The dye could also be incorporated intothe colloidal spheres.

A non-linear dye may also be used, for example one that has asignificantly different refractive index in its excited state comparedto the ground state. This also induces a refractive index contrast underillumination.

The second method of modifying the refractive index contrast in thecolloidal photonic crystal is to apply a variable electric field acrossthe liquid crystal material in the colloidal photonic crystal. In thisconfiguration the glass plates encapsulating the colloidal photoniccrystal are coated with a layer of transparent conductor, for exampleindium tin oxide (ITO), thus allowing an electric field to be to beapplied across the crystal. The orientation of the liquid crystalmolecules can be varied simply by varying the voltage across the cell,thus changing the refractive index contrast and hence altering thediffractive and transmissive properties of the colloidal photoniccrystal. The transmission or the intensity of any diffracted beams canbe changed as a function of the applied electric field.

The colloidal photonic crystals incorporating a liquid crystal materialmay have applications in the protection of electr-optic devices fromboth high and low power laser pulses. The device may also be used as alimiter, a switch or a router by using a high intensity pump beam or thevariable electric field applied to the liquid crystal to change thedirection (redirect into a diffracted order) or block a low intensityprobe beam which may be modulated to carry digital information.

An alternative method of incorporating the liquid crystal material intothe colloidal photonic crystal would be to eliminate the intermediatedispersion medium (octanol) and utilise a liquid crystal dispersionmedium during the initial growth process.

In this arrangement monosized colloidal spheres are suspended in aliquid crystal material at a volume concentration that producesspontaneous crystallisation. The configuration typically comprises 800nm polymethylmethacrylate spheres suspended in K15 nematic liquidcrystal at a volume concentration greater than 49% by volume. Thecolloidal suspension is placed between two glass plates and the sampleis sheared using the enhanced composite shearing technique describedpreviously, forcing the colloid into a single face-centred-cubicstructure.

An alternative and complementary method of eliminating the tendency ofthe colloidal crystal to relax into a twinned face-centred-cubicstructure is to modify the properties of the dispersion medium in whichthe colloidal spheres are suspended. The tendency of the crystal torelax into a twinned face-centred-cubic structure may be overcome byreplacing the traditional octanol dispersion medium with a dispersionmedium capable of undergoing a controllable phase change from a liquidphase to a solid phase. Accordingly, the phase of the dispersion mediumcan be changed from liquid to solid upon demand, enabling the crystal tobe fixed into a single face-centred cubic structure at the appropriatestage in the crystal growth. Suitable dispersion mediums includeadhesives, polymers, and resins. Epoxy-resin that can be cured byexposure to ultraviolet light is of specific interest as a suitabledispersion medium. Alternatively, epoxy-resins which are curable bychemical reaction, temperature or exposure to radiation may be used.

This technique can be used in conjunction with the conventionalone-dimensional shear alignment process or with the compositetwo-dimensional shear alignment process shown in FIGS. 5a or 5 b.

The use of a dispersion medium which is capable of undergoing acontrolled phase change from a liquid phase to a solid phase provides aninexpensive means of producing large area, robust singleface-centred-cubic crystals. Crystals produced by the method exhibitimproved longevity and the crystal structure is essentially permanentFurthermore, the method lends itself to mass production applications.

The colloidal crystal is prepared by dispersing monosized polymercolloidal spheres in an epoxy resin that can be hardened by exposure toultraviolet light. Typically the colloid consists of 800 nmpolymethylmethacrylate spheres at a volume concentration that producesspontaneous crystallisation (greater than 49% by volume). The suspensionis confined between two glass plates (2,3) and pressure is applied sothat the space between the plates is completely filled. 10 micron spacerbeads are used to control the separation of the glass. A controlledlinear one-dimensional shear or a composite two-dimensional shear isapplied to align the crystal into a single face-centred-cubic structure.

The viscosity of the epoxy resin increases the relaxation time of thecrystal compared to the conventional octanol dispersion medium and it ispossible to harden the resin whilst maintaining the singleface-centred-cubic structure by exposing the sample to ultravioletradiation. After exposure to the ultraviolet radiation, the epoxy resinforms a strong interconnecting matrix between the colloidal spheresmaking the crystal structure robust and essentially permanent.

Furthermore, the solid interconnecting matrix facilitates thefabrication of inverse face-centred-cubic structures which exhibit largefull photonic band-gaps. For example, the polymer colloidal spheres maybe removed by dissolving in a suitable solvent. A non-linear material oran active material such as a laser dye may be introduced to fill thegaps left by dissolving the colloidal spheres. This provides a route forproducing photonic crystals in materials that cannot be made directly byconventional methods.

The use of a solid suspension medium also enables the mechanicalproperties of the supporting outer parallel plates to be modified sincethe colloidal crystal produced by this method may be made to beself-supporting. For example, the thickness of the parallel plates whichare used to retain the colloid during the crystal growth phase may bereduced. In this case the mechanical properties of the solid dispersionmedium contribute to the overall strength of the structure.

Altematively, a detachable thin film or membrane could be applied to theinternal surface of one or both of the parallel plates prior to theintroduction of the colloid. After the crystal has been grown and thedispersion medium solidified, the parallel plates could be removed toreveal a robust laminated colloidal photonic crystal film. The film ormembrane could be either rigid or flexible, allowing the laminatedcolloidal crystal colloid consists of 800 nm polymethylmethacrylatespheres at a volume concentration that produces spontaneouscrystallisation (greater than 49% by volume). The suspension is confinedbetween two glass plates (2,3) and pressure is applied so that the spacebetween the plates is completely filled. 10 micron spacer beads are usedto control the separation of the glass. A controlled linearone-dimensional shear or a composite two-dimensional shear is applied toalign the crystal into a single face-centred-cubic structure.

The viscosity of the epoxy resin increases the relaxation time of thecrystal compared to the conventional octanol dispersion medium and it ispossible to harden the resin whilst maintaining the singleface-centred-cubic structure by exposing the sample to ultravioletradiation. After exposure to the ultraviolet radiation, the epoxy resinforms a strong interconnecting matrix between the colloidal spheresmaking the crystal structure robust and essentially permanent.

Furthermore, the solid interconnecting matrix facilitates thefabrication of inverse face-centred-cubic structures which exhibit largefull photonic bandgaps. For example, the polymer colloidal spheres maybe removed by dissolving in a suitable solvent. A non-linear material oran active material such as a laser dye may be introduced to fill thegaps left by dissolving the colloidal spheres. This provides a route forproducing photonic crystals in materials that cannot be made directly byconventional methods.

The use of a solid suspension medium also enables the mechanicalproperties of the supporting outer parallel plates to be modified sincethe colloidal crystal produced by this method may be made to beself-supporting. For example, the thickness of the parallel plates whichare used to retain the colloid during the crystal growth phase may bereduced. In this case the mechanical properties of the solid dispersionmedium contribute to the overall strength of the structure.

Alternatively, a detachable thin film or membrane could be applied tothe internal surface of one or both of the parallel plates prior to theintroduction of the colloid. After the crystal has been grown and thedispersion medium solidified, the parallel plates could be removed toreveal a robust laminated colloidal photonic crystal film. The film ormembrane could be either rigid or flexible, allowing the laminatedcolloidal crystal film to be applied to a second surface and enabling itto conform to the surface (subject to the solidified suspension mediumexhibiting suitable mechanical properties). In a further configurationthe parallel plates could be removed completely after the suspensionmedium had been solidified. This may be facilitated by applying arelease agent directly to the internal surface of one or both of theparallel plates. However, this method would require the suspensionmedium to provide all the mechanical strength for the structure sincethe outer parallel plates would have been eliminated.

Mindful of the potential requirement to produce colloidal photoniccrystals on a large scale, an example of a suitable industrialproduction process is shown schematically in FIG. 7. The compositetwo-dimensional shearing method is retained and is provided in FIG. 7 byshearing the colloid between rollers (30,31). In the example the colloid(1) is inserted between two flexible film membranes (32,33) and thecomposite shear applied between rollers (30,31) which are subjected totranslations (33) with respect to each other. The translations (33)applied to the rollers (31,31) are substantially perpendicular to thedirection in which the colloid is admitted to the rollers. The flexiblefilm membranes may be provided on reels (34,35) to facilitate continuousproduction of colloidal photonic crystal film (37). The dispersionmedium is solidified by any of the aforementioned means as the colloidalcrystal emerges from the rollers. For example, where the dispersionmedium is an ultraviolet curable epoxy-resin, an ultraviolet source (36)may be used.

In an alternative arrangement, separate shearing and rolling means maybe provided to respectively apply the translation and to feed thecolloid through the process. The shearing means may be configured asnon-rotary components.

The examples in the foregoing discussion have concentrated on colloidalphotonic crystals incorporating monosized polymer colloidal spheres.However, alternative materials may be used for the colloidal spheresincluding non-linear optical materials, magnetic materials, metals,semiconductors, doped glass (for example using an active dye dopant),doped polymer (for example using an active dye dopant) and silica.

Additionally, essentially permanent colloidal photonic crystals andinverse face-centred cubic structures may be fabricated using thecomposite two-dimensional shear process without the need to change thephase of the dispersion medium from a liquid phase to a solid phase.

For example, the use of monosized silica colloidal spheres enables thecolloidal photonic crystal to be fixed into a single face-centred cubicstructure at the appropriate stage in the crystal growth. The colloidalcrystal is prepared by dispersing monosized silica colloidal spheres ina dispersion medium which can be removed after the growth of thecolloidal photonic crystal. Typically the colloid consists of silicaspheres suspended in octanol at a volume concentration that producesspontaneous crystallisation (greater than 49% by volume). The suspensionis confined between two glass plates (2,3) and pressure is applied sothat the space between the plates is completely filled. Spacer beads areused to control the separation of the glass. A composite two-dimensionalshear is applied to align the crystal into a single face-centred-cubicstructure.

At the end of the growth process the dispersion medium is removed fromthe colloidal crystal structure and the silica colloidal spheressintered (heated) so that the spheres fuse together. After sintering thesilica spheres form a self supporting structure of colloidal spheressurrounded by an interconnecting matrix of voids. The interconnectingmatrix of voids may be subsequently filled with other materials tomodify the optical properties of the photonic colloidal crystal. Thematerial used to fill the voids may be any of a high index material, ametal, a semiconductor or an active material. The silica spheres may besubsequently removed, for example by etching, to form the inverseface-centred cubic structure which exhibits a wider photonic band gapthan the conventional structure.

In the above process for producing essentially permanent colloidalphotonic crystals and inverse face-centred cubic structures, the silicaspheres may be replaced by alternative materials. For example polymerspheres may be fused together with similar effect. As before, theinterconnecting matrix of voids may be filled with other materials andthe polymer spheres may be subsequently removed to form the inverseface-centred cubic structure.

Further interesting optical effects may be achieved by encouraging thecontrolled growth of multiple large area colloidal crystal domains.Adjacent crystal domains may be tailored to exhibit different opticalcharacteristics.

The composite two dimensional shear process is retained and the internalsurface of one or both of the parallel plates is textured to promote thegrowth of multiple crystal domains. The textured surface may exhibit aspecific pattern to control the growth of the crystal domains into apreferred configuration. The texture or pattern may be applied to theparallel plates by any suitable process including etching and embossing.

The methods described herein to produce single face-centred-cubiccolloidal photonic crystals may be applied to the production of avariety of optical devices including optical filters and limiters. Thereflective properties of the single face-centred-cubic colloidalphotonic crystals also offer further opportunities for exploitation inoptical devices.

What is claimed is:
 1. A method of growing an essentially perfectcolloidal photonic crystal exhibiting a single face-centred-cubicstructure comprising the steps of: i. preparing a suspension ofmonosized colloidal spheres (1) having a volume concentration thatproduces spontaneous local crystallisation in a suitable dispersionmedium, ii. inserting the colloidal suspension into a gap between twosubstantially parallel surfaces (2,3), iii. subjecting the surfaces torelative oscillating motion (5) parallel to their surfaces and, iv.subjecting the surfaces to a series of small linear displacements (6)relative to each other, the displacements being parallel to theirsurfaces and in two dimensions, comprising the sequence of applying alinear displacement to one of the surfaces with respect to the othersurface, rotating the direction in which the linear displacement isapplied to the surface by substantially 120 degrees in a single constantdirection and applying a further linear displacement to the surface, thesequence being repeated until the colloidal photonic crystal has beenpurified into a single face-centred-cubic structure.
 2. A methodaccording to claim 1 wherein the dispersion medium is changeable from aliquid phase to a solid phase in order to fix the colloidal crystallinestructure.
 3. A method of growing an essentially perfect colloidalphotonic crystal exhibiting a single face-centred-cubic structurecomprising the steps of: i. preparing a suspension of monosizedcolloidal spheres (1 ) having a volume concentration that producesspontaneous local crystallisation, in a dispersion medium that ischangeable from a liquid phase lo a solid phase in order to colloidalcrystalline structure ii. inserting the colloidal suspension into a gapbetween two substantially parallel surfaces (2,3), and iii. subjectingthe surfaces to relative oscilating motion parallel to their surfaces(5).
 4. A method according to claim 1 wherein the magnitude of the smalllinear displacements applied to the surfaces is substantially equal tothe product of the diameter of the colloidal spheres and the number ofcrystalline layers in the crystal.
 5. A method according to claim 1wherein the surfaces are displaced with respect to each other in anequilateral triangle.
 6. A method according to claim 1 wherein theminimum volume fraction of monosized colloidal spheres is 0.49.
 7. Amethod according to claim 1 wherein the radius of the monosizedcolloidal spheres is in th range 0.01 pm to 100 pm.
 8. A methodaccording claim 1 wherein the radius of the monosized colloidal spheresis in the range 0.05 pm to 10 pm.
 9. A method according to claim 1wherein the material used for the colloidal spheres is at least one of apolymer, a non-linear material, a magnetic material, a metal, asemiconductor, glass doped with an active dye, polymer doped with anactive dye, silica.
 10. A method according to claim 9 wherein thecolloidal spheres are polymethylmethacrylate.
 11. A method according toclaim 1 wherein the material used for the dispersion medium is at leastone of an adhesive, a polymer, a resin, a non-linear optical maternal,an active optical material, octanol.
 12. A method according to claim 11wherein the active optical material used for the dispersion medium is aliquid crystal material.
 13. A method according to claim 1 wherein thedispersion medium is subsequently removed from the colloidal photoniccrystal to leave a structure comprising colloidal spheres surrounded byan interconnecting matrix of voids.
 14. A method according to claim 13and further comprising the subsequent step of introducing a substitutematerial into the interconnecting matrix of voids surrounding thecolloidal spheres.
 15. A method according to claim 14 wherein thesubstitute material is at least one of a metal, a semiconductor, anon-linear optical material, an active optical material.
 16. A methodaccording to claim 15 wherein the substitute active optical material isa liquid crystal material.
 17. A method according to claim 15 andfurther comprising the subsequent step of removing the colloidal spheresfrom the substitute material.
 18. A method according to claim 12 andfurther comprising the step of adding to the colloidal photonic crystalmeans for applying an electric field to the liquid crystal material. 19.A method according to claim 2 wherein the material used for thedispersion medium is at least one of an adhesive, a polyner, a resin.20. A method according to claim 1 wherein the dispersion medium is alepoxy resin and further comprising the subsequent step of curing theresin to form a solid interconnecting matrix between the colloidalspheres.
 21. A method according to claim 20 wherein the curing processincludes at least one of exposure to electromagnetic radiation, exposureto ultraviolet radiations chemical reaction, elevation of temperature.22. A method according to claim 2 wherein at least one of thesubstantially parallel surfaces (2,3) comprises a substantially flexiblemembrane.
 23. A method according to claim 2 wherein the series of smalllinear displacements (6) is applied to the surfaces by rolling means(30,31) to produce bulk colloidal photonic crystal film (37).
 24. Amethod according to claim 2 and further comprising the intermediate stepof applying a detachable membrane to the internal face of at least oneof the parallel surfaces prior to introducing the colloidal suspension.25. A method according to claim 1 wherein the internal surface of atleast one of tie parallel surfaces is textured to promote the growth ofmultiple crystal domains.
 26. A method according to claim 1 wherein therefractive index of the dispersion medium is substantially differentfrom the refractive index of the colloidal spheres.
 27. A methodaccording to claim 26 wherein the refractive index ratio between thecolloidal spheres and the dispersion medium is greater than two.
 28. Amethod according to claim 2 and further comprising the subsequent stepof removing the colloidal spheres from the solidified dispersion medium.29. A method according to claim 28 and further comprising the subsequentstep of introducing a substitute material into the voids in thesolidified dispersion medium.
 30. A method according to claim 29 whereinthe substitute material is at least one of a non-linear opticalmaterial, en active optical material or a laser dye.
 31. A methodaccording to claim 1 wherein the two surfaces are concentricallycylindrical (20,21).
 32. An essentially perfect, singleface-centred-cubic colloidal photonic crystal produced by the method ofclaim
 1. 33. An optical notch filter having a colloidal crystalaccording to claim 32 wherein the colloidal sphere radius and refractiveindex of the dispersion medium are selected to co-operate to reflect atleast one specific wavelength and to transmit other wavelengths.
 34. Anoptical device having a colloidal crystal according to claim 32 andfurther comprising: a liquid crystal material, and means for applying anelectric field to the liquid crystal material wherein a variable voltageis applied to the liquid crystal material to change the refractive indexcontrast between the liquid crystal material and the colloidal spheres.